A material for heat dissipating parts of semiconductor devices is required to have a higher thermal conductivity. Moreover, in order to prevent the connection from being thermally broken down by thermal expansion and contraction, such a material is required to have a thermal expansion coefficient equivalent to that of a semiconductor material that is a heat source. Furthermore, in application to portable semiconductor devices, more emphasis is placed on lightness in weight.
In recent years, for the above purposes, a composite material using a graphitic carbon fiber has attracted attention as a material having a higher thermal conductivity, lower thermal expansion coefficient and lower density than metal. For example, attention has been focused on composite materials using a graphitic carbon fiber having a random structure represented by a polyacrylonitrile (PAN)-based carbon fiber, a graphitic carbon fiber having a radial structure represented by a pitch-based carbon fiber, and a graphitic carbon fiber having a tubular structure represented by a vapor grown carbon fiber (VGCF).
However, the high thermal conductivity of graphite is exhibited in a direction parallel to a hexagonal network structure (graphite structure) composed of carbon atoms in the crystalline structure. Meanwhile, the thermal conductivity of graphite is low in a direction which crosses or is perpendicular to the graphite structure (in a direction which goes through the hexagonal network). In any of the aforementioned graphitic carbon fibers, the graphite structure develops in a length direction of fiber. Accordingly, a high thermal conductivity is obtained in the length direction of the graphitic carbon fiber. Nevertheless, in a thickness direction of fiber, any of the fibers has portions which cross or are perpendicular to the graphite structure. For this reason, the thermal conductivity in the thickness direction of the fiber is low compared with that in the length direction of the fiber.
Recently, with the advancement in semiconductor miniaturization, studies has been conducted on a structure in which heat generated from a semiconductor is dissipated from a large heat dissipation area by diffusing the heat two-dimensionally, instead of a structure in which the heat is transmitted in one direction and then dissipated in a region different from the region where the heat is generated. Such a structure has been studied particularly for notebook PCs, portable communication devices, and the like, in which locally high temperatures are not favorable.
On the other hand, the composite material using a graphitic carbon fiber is unsuitable for diffusing heat two-dimensionally due to the anisotropy of the thermal conductivity of the carbon fiber used therefor. Except for a case where the carbon fiber is aligned in one axial direction, the currently-achieved highest thermal conductivity of the composite material is lower than that of silver which is a metal having the highest thermal conductivity (thermal conductivity=approximately 430 W/m/K).
Meanwhile, a graphite scaly powder has drawn attention due to its high self-lubricity, and its composite material made particularly with copper is widely used in sliding components of a brush for direct current motor and the like (see Patent Citations 1 and 2). In these sliding components, dispersibility of a graphite powder into metal gains importance to efficiently exhibit the self-lubricity. Few studies have been made on the thermal conductivity and the orientation of the graphite powder to obtain a high thermal conductivity. Moreover, when these components are manufactured, a pressureless sintering method is widely adopted. No application of pressure during sintering and low affinity between copper and the surface of graphite result in that these materials are contained at low density.
Moreover, there has been also studied a method to obtain a composite material for a sliding component by sintering a mixture including a flattened copper powder in addition to a graphite scaly powder and spherical copper particles (see Patent Citation 3). However, the use amount of the flattened copper powder is up to 40% by weight relative to the whole amount of copper. Additionally, the flattened copper powder is used to segregate copper on the surface of the composite material for the sliding component. The resultant orientation of the graphite scaly powder is not mentioned at all.    Patent Citation 1: Japanese Patent Laid-Open No. H07-207253 (1995)    Patent Citation 2: Japanese Patent Laid-Open No. H08-331811 (1996)    Patent Citation 3: Japanese Patent Laid-Open No. 2004-300485    Non-Patent Citation 1: P. M. Adams, H. A. Katzman, G. S. Renick and G. W. Stupian, “CHARACTERIZATION OF HIGH THERMAL CONDUCTIVITY CARBON FIBERS AND A SELF-REINFORCED GRAPHITE PANEL”, Carbon, Vol. 36, No. 3, pp. 233-245 (1998)